A common goal in loudspeaker system design is to achieve a constant beamwidth, or directivity, in both the horizontal and vertical planes over the loudspeaker's operating frequency range. Often the desired beamwidth in the horizontal plane is kept relatively wide (70 to 100 degrees) in order to direct sound uniformly over the width of the room or audience from a single acoustic point. Within this range, the desired horizontal beamwidth will depend on the width of the room, reflective properties of walls in the room, and the location of the loudspeaker. For example, a horizontal beamwidth at the low end of the range will avoid destructive acoustic reflections from nearby walls in the room, whereas a horizontal beamwidth on the high end of the range can be used where the walls of the room are acoustically absorbent or not in destructive proximity to the sound field. In either case, it is desirable to create a beam of sound having a horizontal beamwidth that meets the room conditions but yet is sufficiently large to cover the entire audience to its outer edges over the operating frequency range of the loudspeaker.
To accommodate different room conditions two different horizontal beamwidths are sometimes offered in a loudspeaker system. This is accomplished by providing two different waveguide horns, either of which can be installed into the loudspeaker enclosure. In that case the waveguide horns are kept similar in all respects except for the shape of the horizontal expansion of the horn. The horns with different horizontal expansions produce polar patterns with different horizontal beamwidths, and can be designed to produce horizontal beamwidths in a range between 70 to 100 degrees.
The desired beamwidth in the vertical plane is usually a much smaller angle than the horizontal. This is because the depth of the seating for an audience as seen by the loudspeaker is normally relatively shallow and only requires a relatively small vertical beamwidth angle to achieve the desired coverage. A vertical beamwidth of 40 to 50 degrees is often desired to concentrate acoustic energy to the audience and prevent acoustic energy from spreading elsewhere in the room.
Beamwidth is typically defined as the angle at which the magnitude of the acoustic pressure wave is 6 dB lower than the measured pressure on-axis to the loudspeaker. It is understood by those of ordinary skill in the art that, if the pressure has not attenuated more than 6 dB over a range of seats, the sound will generally be observed as similar to the on-axis response, provided a large portion of the frequency band attenuates equally. This is considered uniform coverage in the field of acoustics.
To obtain this goal, the beamwidth must be kept constant over the widest possible range of frequencies. In a two way loudspeaker system a crossover is used to split the full range audio signal into a high frequency channel and a low frequency channel. The high frequency channel uses a waveguide horn to control the vertical and horizontal beamwidth. To obtain a wide range of frequencies where the beamwidth is kept constant, the waveguide horn is made large to work well at long wavelengths and the crossover frequency is set at a relatively low frequency. A dimension of 24 inches by 48 inches at the face of the horn is not uncommon to obtain directional control down to 1000 Hz. When the angle of beamwidth control is small, such as 40 degrees as required in the vertical plane, the horn must be made physically deeper and larger to obtain control at the lower end of its operating frequency range.
The above-mentioned approaches to achieving beamwidth control in a waveguide horn have significant drawbacks certain applications, and particularly in cinema applications. In cinemas applications the loudspeakers are placed behind a cinema screen, and using loudspeakers having large or deeper waveguide horns can detrimentally affect the beamwidth and frequency response of the loudspeaker system due to waves reflecting back and forth between the screen and the surface of the horn. Reflections from the back of the cinema screen propagate back onto the walls of the horn causing the sound waves to reflect in undesirable directions. Sometimes referred to as beam spreading, this phenomenon degrades beamwidth control. The present invention overcomes the drawbacks of these previous approaches by permitting the use of a smaller horn behind a cinema screen while maintaining the desired beamwidth control in the vertical and horizontal directions. Because of this, the affect of the screen on the system's frequency response and beamwidth control over a range of frequencies is substantially reduced or eliminated.